test: Skip TEST-06-SELINUX early if not on fedora/centos

Other distributions may be able to install selinux
but they are not expected to use it.

The distribution is tested rather than whether selinux is enabled
because it is expected to work on CentOS and Fedora
and we want it to fail noisily.

test/units/testsuite-06.sh

@@ -3,6 +3,12 @@
 set -eux
 set -o pipefail
 
+. /etc/os-release
+if ! [[ "$ID" =~ centos|fedora ]]; then
+    echo "Skipping because only CentOS and Fedora support SELinux tests" >>/skipped
+    exit 77
+fi
+
 # Note: ATTOW the following checks should work with both Fedora and upstream reference policy
 #       (with or without MCS/MLS)